Method of producing concrete structures with a surface protection and a concrete structure produced in accordance with the method

ABSTRACT

Method of producing concrete structures with a surface protection layer and a concrete layer. The method includes moulding the concrete layer and the surface protection layer using a &#34;wet in wet&#34; process. The surface protection layer preferably consists of a cement allied mortar having a low water binding average ratio and a binding agent strength above 70 Mpa. The concrete layer which preferably consists of a concrete having a lower binding agent strength than the surface protection layer, is preferably an air entrained ballast concrete with dense structure and a strength in the range of 10-20 Mpa. The surface protection layer is cast in the bottom of a mold, and the concrete layer is cast above the surface protection layer. During desiccation of the material, the surface protection layer shrinks less than the concrete layer, so that pressure strains are created through deformation differences in the surface protection layer and concrete layer.

The present invention refers to a method of producing concretestructures with a surface protection on the underlying concrete, bycasting the latter and at least a surface layer substantially "wet inwet", as well as a concrete structure with a surface protection in formof a surface layer, which is integrated with the underlying concrete byhaving the latter and the surface layer made substantially through a"wet in wet"-process.

BACKGROUND OF THE INVENTION

Concrete structure surfaces and surface layers have a significantimportance for the technical lifespan of the structure. They completelydetermine the look or, in other words, are essential for the aestheticquality of the concrete structures. Other technical characteristics thatare bound to surface are wearing qualities, resistance against highpoint loads and impact resistance. For outdoor structures, thecharacteristics relate to those surfaces that are broken down by frostinfluence, erosion and chemical attack.

It is a common procedure to protect a material through some type ofsurface treatment. Steel and wood are examples of such materials, whichpermanently and initially, need to be provided with surface protection.Concrete however is not seen as needing to be protected. The philosophyis that concrete, right from the beginning, must be resistant enoughagainst those influences to which the structure is exposed too. Onecomposes concrete during proportioning so that set conditions arefulfilled. This is controlled during pretesting. The requirements arebased on the understanding that a concrete structure has to be given adetermined technical life span, for example 100 years. It is well knownthat it is quite difficult to succeed with the casting of concretelayers, and in particular such thin ones, on an already hardened andaged concrete. Differences in deformation process such as shrinkage andevolution of the adherence between the underlying concrete and thesurface layer are probable causes of bad results.

The ground to concrete structures life-span lies in the protection ofthe reinforcement. The concrete that covers the reinforcementconstitutes, as it is well known, the corrosion protection. The choiceof concrete is guided almost exclusively by the characteristics that arerequired for the concrete to provide satisfactory reinforcementprotection. The strength requirement for the structures, which areproduced with present standards is about to be automatically fulfilled.The concrete is provided with a strength, which in some cases widelyexceeds what is needed for the carrying capacity. With presentproduction techniques, one is forced to choose the same high qualityconcrete throughout a whole concrete structure even if it is only in thesurface layer that the highest set requirements are required.

Through SE-B-368 599, a procedure for producing building plates of lightconcrete with a front layer of normal concrete is already known, wherethe light concrete layer, which has a bulk density of 625 kg/m³, isexposed to a low pressure so that the excess water is sucked out beforethe front layer is mounted. The low pressure treatment has the purposeto prevent shrinkage during the hardening and to let the tensionsappearing between the layers remain therefore slight.

Through the Swedish publication 302 911, a procedure for producingbuilding blocks and plates with a resistant surface layer is also known.This consists of many layer parts, a part nearest the normal concretecomprising decomposed mineral/glass-fibre particles and larger grains ofcement, cinder and sand and that intermediate layer forms an efficientfilter mat, which only lets those very smallest particles of thecomponents cement, cinder and sand, slip through, said componentsbuilding the layer part located nearest to the casting mould. Bothsurface layer parts are left to be bound, which can be done under vacuumor heat, so that at least a partly bound bottom layer has been obtainedbefore normal concrete is poured over the surface layer. Since thesurface layer must be of low viscosity, it is probably a necessity toachieve a stabilization of the bottom layer, i.e. until it startsbinding, which usually happens within 1-3 hours. A firm consistency isrequired for the normal concrete during its casting over the surfacelayer to avoid forcing its way through or pushing it out. Since thedensity in the different layers is rather like each other, no big stressunder compression arises in these layers.

SE-B-321 178 describes a method to produce building elements withdifferent density in different cross section parts using the samebinding agent. According to this publication, porous light concreteelements with a dense front surface are produced by placing in a castingmould against the mould surface slices of asbestos fibres and bindingagent that are partly steam hardened and that space behind the slices isfilled with a pore building light concrete mass. After the lightconcrete has bound up and solidified in the mould, it is steam hardenedin an autoclave at a temperature that is higher than 150° C. Theproduction of pre-prepared slices as well as the autoclave processing ofthe entire building block make the manufacture more expensive and makethe procedure applicable only to prefabrication.

According to U.S. Pat. No. 3,286,418, composite concrete plates areproduced, where the surface layers are solidified at an increasedtemperature before the central core of light concrete is poured. Toachieve a better transition between the surface layer and the centralcore, the contact surface is coated with an adhesive polish. No type ofstress under compression is produced between the different layers.

The progress of concrete technology during the last two decades hasenabled an appreciable widening of concrete use. There is hardly anymaterial, which can be provided in as large a width as concrete.Compression strengths of 3-300 Mpa and densities of 300-3000 kg/m³ canexemplify this assertion. On top of that, development of reinforcementusing fibre techniques and modification with polymers is in progess. Thebinding agent of Portland cement can even be entirely changed topolymers.

SUMMARY OF THE INVENTION

The object of the invention is to produce different types of surfacelayers on concrete during manufacturing. The surface layers shall havethe possibility to completely cooperate with the underlying concrete,shall reinforce the protection of structures in various enviromentalconditions, which have changed with time, shall in special cases be ableto reduce the requirement on the covering concrete layer, and shall asextra protection further improve the durability in important structureparts even in an extremely aggressive environment. This has beenachieved through a method of producing concrete structures with asurface protection on the underlying concrete, by casting it and atleast a surface layer substantially "wet in wet".

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial perspective view of a concrete structure accordingto the invention having a surface layer and a concrete layer.

The invention is based consequently on giving rise to pressure strainsin the surface layer in concrete structures. These pressure strains arecreated by deformation differences in the surface layer and theunderlying concrete. The deformations are caused by shrinkage and dryingshrinkage that arise after the hardening. The end shrinkage can beregarded as being reached when a moisture balance has been set with thesurrounding. The prerequisite is consequently that the surface layer andthe underlying concrete material have different shrinkage magnitude andthat the shrinkage of the surface layer is smaller than the underlyingconcrete. This can generally be fulfilled when the surface layer is madeof mortar or concrete with high strengths or of other material, whichdoes not have after-shrinkage and if the underlying concrete is of thelightweight type. Lightweight concrete has a low elastic modulus andtherefore gives a limited obstacle for the shrinkage. It is a necessarycondition that the so-called relaxations that are generated between thesurface layer and the existing underlying concrete must be able to becarried out without the separating of the surface layer. This problemcan be solved by resorting to particular actions in the transition zoneassociated with the casting.

Surface layers and concrete cast "wet in wet" will eventually generatepressure strains in the surface layer due to the influence ofdifferences in drying shrinkage between the surface material and theunderlying concrete. To get pressure strains in the surface layer thatis aimed at, it is required that the surface material have a smallershrinkage than the underlying concrete. Pulling strains appear at thesame time in the underlying concrete. The magnitude of the tensions isdetermined by both the elastic modulus of the mat and the thickness ofthe surface layer.

The pressure strains in the surface layer are approximately 10-15 Mpaand pulling strains in the underlying concrete 1/10 thereof. One canroughly come up to this under the condition that one supposes thatbending of the underlying concrete does not occur, that the shrinkage isapproximately twice as big in the underlying concrete, that the elasticmodulus in the underlying concrete is 1/5-1/7 of the one in the surfacelayer and that the operative thickness in the underlying concrete isabout 10 times as large the surface layer. If the bending is taken intoconsideration, all tensions are reduced as the constraint is reduced.The maximum pressure strain in the surface layer is reduced with somemore than 20% and the maximum pulling strain in the underlying concretewith less than 10%. The biggest deflection is approximately 1/400 of thespan when this one is 1.2 m.

Within the surface layer, the shrinkage strains will have a differentmagnitude. The difference is caused by the separation of the aggregateparticles so that different aggregate volume shares arise within thelayer, with a low percentage in the upper part. When the volumepercentage is reduced from 0.7 to 0.35, i.e. halved, the shrinkageincreases about four times. The calculation is done using Pickett'sformula, which follows: ε=ε_(p) ·(1-g)^(m), where εp is the shrinkage ofthe water-cement paste, g is the aggregate volume share and m=1.7 forquartz material and the like. The results can then be that the lessshrinking operative thickness of the surface layer becomes thinner, thepressure strains bigger and the bending smaller. Through intentionallyraised sedimentation of aggregate particles in the surface layer, asuccessive increase of the shrinkage and a reduction of the pressurestrains in the direction of the underlying concrete appear.

The most important thing to avoid on one hand is a pulling strength inthe mortar stage that is exceeded in the surface layer, and on the otherhand to facilitate that already built cracks get a reduced crack widthor even the possibility to be closed. Lightweight concrete elasticmodulus is 6000-7000 Mpa and its shrinkage (end shrinkage) at 50%relative air humidity is at least 0.9%.

There are several reasons to produce pressure in the outer layer on amaterial and in particular a brittle material like concrete. A directparallel is the effect of hardening on glass, which also is a brittlematerial. Defects in the surface such as scratches or microcracks arequite decisive for the material to break through the very high strainsthat arise in the crack ends and the strength with regard to the outerstress. The hardening process gives pressure strains in the glasssurface layer. One counteracts the pulling strains and diminishes therisk that so-called brittle break occurs.

In concrete material, the strains in the surface layer will not becomeas big as in a glass surface layer where strains of up to 140 Mpa canarise after the hardening. The goal is however not the same withconcrete material even if similarities are found. For concretestructures, the following discussed below can be achieved with thinsurface layers.

Common requirements that are generally found and can be set on surfacesare the following:

Good resistance against attack from the surrounding environment.

Good chemical resistance.

Resistance capacity against temperature differences and temperatureshocks.

Good wearing qualities.

Pore and crack-free surfaces.

Flexibility in colour and structure.

Technical effects: High strength in general, good resistance againstphysical/chemical influence, such as salt-frost bursting, high wearingqualities, great resistance against impact and high point loads,fulfilling of high sanitary requirements, great resistance against acidattack and salt influence and low permeability with regard to chloridesand carbon dioxide are usual characteristics that nowadays are actualfor concrete structures.

Aesthetic effects: Great freedom to colour surfaces with longdurability, among others by pigment inking, good possibilities to createsurface structures, such as reliefs, smooth and even surfaces withpossibly high lustre (without grinding) and, in the manufacturingprocess, to create flexible surfaces with the mosaic technique.

A material that is particularly adapted to combination is, for surfacelayers, cement bound mortar with low water binding agent ratio,equivalent to a binding agent strength in general over 70 Mpa. Asunderlying concrete, lightweight concrete with dense structure hasappeared particularly appropriate, for example the lightweight concretewith a density in the range 800-1500 kg/m³ mentioned in SE-B-8305474-2.This concrete type is a structure concrete with approximately half thedensity of normal concrete, strengthen in the area 10-20 Mpa, containshigh air entrained volume in cement paste, has hydrophobiccharacteristics and is open for diffusion with regard to water vapour.Drying shrinkage is further about three times bigger than for thehigh-test concrete with particular high strength, which can be chosenfor the surface layer. It is possible to use concrete types with usualheavy aggregate material as the underlying concrete with a density ofabout 2300 kg/m³.

Materials other than the cement bounded type can be chosen, for examplethermosetting resin such as epoxy and urethane as well as combinationsbetween polymers and cement. These are of the type that corresponds tomodified polymer concrete in which hydraulic binding agents cooperatewith polymer dispersions, for example based on acrylic and styrenebutadiene.

The surface layer, consisting of merely thermosetting resin, should bechosen with little thickness, from 100 μm to 1-2 mm. The advantage offixing the polymer layer in connection with the concrete casting is thatthe polymer becomes completely tight. Pore formation that frequently forchannels in polymer layer occurs when the layer is applied on hardenedconcrete surfaces. Polymer types must be compatible with non hardenedconcrete.

The hydraulic binding agent is based partly on Portland cement, whichcan be regarded as calcium silicate-cement and partly on calciumaluminate-cement. Particularly with Portland cement, different agentsand supplemental material are added to change the characteristics inboth the fresh and the hardened material. In the modern concrete thereis foremost the binding agent, which has changed by means ofcombinations of additive and supplemental material. Additives arecompounds when used in a small amount can change the chemical andphysical characteristics. To these additives belong ,for example,dispersing, also referred to a wetting and water reducing, accelerating,retarding air pore creation, tightening and hydrophobing. Supplementalmaterial are those materials, which cooperates with Portland cement as abinding agent, such as puzzoluna (microsilica and fly ash) and latenthydraulic binding agent (granulated slag).

Instead of cement paste with low water-cement ratio and providing littledrying shrinking after a longer drying period one can use expansivePortland cement in the upper layer. This type of cement providesswelling in hardened condition during the first 14 days by supplyingextra water on the surface. In that way compressive stress is built upin the upper layer even during the first curing time before drying outis developed. The objective is achieved regardless of drying out or whenthe drying out can occur, and compressive stress is caused in the toplayer. Swelling was achieved in these cement types by establishingettringit (a mineral created under swelling conditions) during thehydration process or similar hydrations products. The size of expansionis regulated by adding expansive cement or expanding component admixtureto Portland cement. There are even other system, which can developswelling together with Portland cement such as admixtures of gypsum ingreater amount.

Because the surface layer has relatively little thickness the particlesize of the aggregate material is limited. Maximum aggregate size shouldnot outreach half the thickness of the layer. If the largest aggregatesize is 2 mm the thickness of the layer as a rule should be at least 4mm. The minerals or the kind of rocks that is suitable for the surfacelayer is the same as normally used in usual concrete, with same demandwith regard to for example durability, strength and wear resistance. Forthe surface layer it is important that the colour of the aggregatescooperates with the colour, which the surface layer shall have. Normallythe aggregates shall be light when the surface layer shall have lightnuance. Pigment added to the binding agent and thereto adapted aggregatematerial enables big variation in the colour of the surface layer.

The structure of the surface layer is determined by the form material.The casting becomes a copy of the surface of the form. When using mostof the form material for concrete casting the form material must becovered by some type of release agent, i.e., form oil. In many cases theform oil has negative impact on the concrete surface, such asdiscolouration and a problem in connection with later surface treatment.The polymer modified binding agent and the particular layer ofthermosetting plastics demand a special release agent. The choice ofform material in these cases is important. A general requirement is toavoid all forms of form oils, which can influence the surfaces.

At the production of building unit conventional concrete castingtechnique is used. The surface layer is moulded in a lying form and isvibrated, e.g. by form vibrators, to compress the material, to obtain aneven thickness as well as to drive out eventual air-bubbles. By thelayer thickness being small, one obtains entirely pore free surfaces.Underlying concrete, for example light concrete, is suitably mouldedimmediately after that the surface layer laying is finished.

The surface layer and the underlying concrete can also be reinforcedwith conventional material, for example fibre reinforcement and theusual reinforcement in the underlying concrete. During the casting ofthe under concrete layer, a problem with penetration of the underlyingconcrete through vibrating can arise, specially at barrel surface layer.When the concrete material, which is chosen as underlying concrete hasapproximately half of the density of the surface layer material, thepenetration is avoided. Further, extra fibres can be scattered on thenewly moulded surface layer, which both reinforces and constitutes thecarrying surface for the weight, which is applied from conventionalreinforcement without using a distance spacer to ensure a determinedcoat layer thickness. Distance models provide visible marks on thesurface and risk making a leak arise.

The surface layer is a compound meeting the valid requirement, forexample durable surface, which resists high concentrated loads. Thesurface can also be given a determined colour. By using so-called superwetting agents, mortar with very loose consistency and with the abilityto float out during the casting can be obtained. The ballast particlesin the mortar can be allowed to separate so that the volume share ofparticles in the bottom are higher than the average for the layer. Thisis advantageous for surface hardness and durability. The top of thelayer instead becomes generally devoid of ballast particles. Thisseparation layer, which mainly consists of paste, gives a successivetransfer to the top concrete without a sharp border between the twomaterials. The tension gradients in the transition thereby becomesmaller. In extreme cases, this layer can be a blocking layer increasingthe impact resistance by energy absorption and energy distribution. Adouble layer can be moulded in such a case and will reinforce the effectof the middle layer and increase the resistance to blows and impacts.Some type of polymer dispersion can be added in the first mortar layeror fibre in the blocking layer. The same basic technique is the basisfor the structure of bulletproof glass.

The surface layer, where particular requirements exist for fireresistance, should have as little steam resistance as possible, to avoidsteam explosion and early splitting of the surface layer. Air porebuilder in the mortar, open the ballast material by gas or cavitymortar, i.e. mortar with deficit of cement paste can be a possiblesolution when dense surface layer must be avoided.

Referring to FIG. 1, a concrete structure according to the invention isshown. The structure comprises a surface layer 1 and a concrete layer 2.Layer 1 is cast on the bottom of a mould, and has a binding strength ofat least 70 MPa. Layer 2 is cast above layer 1 in the mould, and has adense structure. Layer 2 preferably is comprised of air entrainedballast 3 with a strength of 10-20 MPa.

Examples of material and compound for mortar in the surface layer are asfollows:

1. Portland cement: sand. d_(max) =2 mm. (per weight)=1:1-5water+air-cement ratio=0.2-0.5 wetting agent/floating agent(naphthalene, melamine, acrylic or lignosulphonate based) 0.2-4% ofcement amount.

    ______________________________________                                        2.     Portland cement  1.0 (per weight)                                             microsilica      0.05-0.40 (per weight)                                       sand             1-5 (per weight)                                             floating agent   1.5-4% of cement amount                                      water binding average value                                                                    0.25-0.5                                              3.     Portland cement  0.2-0.8 (per weight)                                         grounded granulated cinder                                                                     0.8-0.2 (per weight)                                         sand             1-5 (per weight)                                             floating agent   0.5-2% of cement amount                                      water binding average value                                                                    0.25-0.5                                              4.     Portland cement  0.60-0.95 (per weight)                                       fly ash          0.40-0.05 (per weight)                                       sand             1-5 (per weight)                                             floating agent   1.5-3% of cement amount                                      water binding average value                                                                    0.25-0.5                                              ______________________________________                                    

5. The above, in combination with microsilica and fly ash, amongthemselves having all mutual contents possible. Other types of pozzolanasuch as trass or Santorini soil (a soil or earth which is a volcanictuff coming from the Greek island of Santorini) can also be used.

6. The above, with colour pigment from 0.05-0.25 weight percent ofcement amount or cinder amount. For light colors white Portland cementand white microsilica should be chosen. Generally, cinder gives lightmortar and can therefore be included without problem to obtain lightcolors. Some agents can cause colour changes, such as naphthalene andlignosulphonate types.

7. Ballast material with other densities than normal quartz. The amountis converted to corresponding volumes without other changes in thecompound.

8. Polymer dispersions are added to the recipe examples 1-7 with apolymer amount that corresponds to 2-15% of the weight binding agent.

9. Grounded granulated cinder, which activates with alkalis, sulphatelime etc. can constitute the binding agent, specially the increasedchemical resistance (acid attack) and block against penetration ofchloride is intended to be achieved.

Examples of surface layer of polymers or thermosetting plastics areepoxy, urethane and polyester. To combine these with fresh concrete,which consequently has not hardened, the same principles should be validfor the function, namely that the surface layer should have fulladhesion in the mortar state against the concrete and that pressuretensions are developed. Since shrinkage of the polymer layer above allis bound to the polymerization, the shrinking in later phase isinsignificant. Furthermore, epoxy has hardly any shrinkage at thepolymerization unlike, for example polyester types.

Examples of the air entrained ballast concrete is found documented inSE-B-8305474 (publication number: 453 181).

Practical samples of the concrete structures produced according to theinvention have shown that a complete cooperation between the surfacelayer and the underlying concrete is obtained. The surface layer of theconcrete surface was porous and crack free and exhibited good resistancecapacity against temperature differences and temperature shocks.

The invention has been described in detail with particular emphasisbeing placed on the preferred embodiment, but modifications andvariations within the spirit and scope of the invention may occur tothose skilled in the art to which the invention pertains.

We claim:
 1. A concrete structure comprising:a surface layer comprisedof cement allied mortar with a low water binding average ratio and abinding agent strength of at least 70 Mpa, and of separated ballastparticles; and a concrete layer above the surface layer, said concretelayer being comprised of a concrete layer having a lower binding agentstrength than the surface layer, said strength being approximately 10 to20 Mpa, said concrete layer having an air-entrained ballast and having adense structure;wherein a part of said surface layer adjacent to saidconcrete has a lower volume percent of ballast than the part of saidsurface layer facing away from said concrete layer.
 2. A concretestructure as defined in claim 1, wherein the surface layer has adesiccation shrinkage that is less than desiccation shrinkage of theconcrete layer, whereby pressure strains through deformation differencesare built in the surface layer and the concrete layer.
 3. A method forproducing a concrete structure having a surface layer and a concretelayer, said method comprising:casting the surface layer in the bottom ofa mould, said surface layer being comprised of cement allied mortar witha low water binding average ratio and a binding agent strength of atleast 70 Mpa; casting the concrete layer in the mould above said surfacelayer, said concrete layer being comprised of an air entrained concretehaving a lower binding agent strength than the surface layer, saidstrength being approximately 10 to 20 Mpa and said concrete layer havinga dense structure; desiccating the surface layer and the concrete layer,wherein said surface layer shrinks less than the concrete layer tocreate pressure strains through deformation differences between thesurface layer and the concrete layer and separating ballast particles insaid surface layer to render a part of said surface layer adjacent tosaid concrete layer to have a lower volume percent ballast than the partof said surface layer adjacent to the mould.
 4. A method as defined inclaim 3, wherein a blockage layer is arranged between said surface layerand said concrete layer, wherein said blockage layer is comprised of thesame general compound as said surface layer.
 5. A method as defined inclaim 3, herein desiccation shrinkage of the concrete layer isapproximately twice as large as desiccation shrinkage of the surfacelayer.